This invention relates to modified polyolefins, especially a class of polyolefins useful as engineering thermoplastics that are known as "thermoplastic olefin" polymers, or "TPOs".
Vehicle manufacturers are increasingly looking to engineering thermoplastic materials as a lightweight, cost-effective replacement for metal and other polymer materials in many applications such as body panels, bumpers, doors, interior trim, etc. In addition to being lightweight and relatively low in cost, engineering thermoplastics provide increased design flexibility, favorable thermal and mechanical performance, and are recyclable.
One common class of engineering thermoplastics is thermoplastic polyolefins (TPOs). TPOs are blends of a polypropylene and an elastomer such as an ethylene-propylene copolymer, and often contain other additives such as fillers and pigments.
Polyolefins, generally, and TPOs, in particular, are nonpolar materials. Because they are nonpolar, many paints and adhesives do not adhere well to TPOs, because those paints and adhesives tend to be polar materials. Accordingly, it has been necessary to treat TPO parts prior to painting or gluing in order to increase the adhesion of the paint or of the adhesive to the TPO.
Treatments to enhance adhesion to TPOs fall into two main types. The first type is a surface treatment, which comes in several forms. The surface can be oxidized to increase its polarity, such as by a plasma treatment, corona discharge or surface etching. Another form of surface treatment is to apply a thin layer of a water- or solvent-based adhesion promoter or primer to the surface of the TPO. However, these surface treatments tend to significantly increase the cost of using TPO parts.
The second main type of TPO treatment involves modifying the TPO matrix itself so that when a part is molded from the TPO resin, no additional treatment steps are needed for adequate adhesion to coatings or adhesives. This second approach generally involves incorporating chemicals having polar groups into the TPO. A common approach involves the use of a modified polyolefin additive that has polar groups grafted onto a polyolefin backbone. One way of doing this is to graft or copolymerize an unsaturated anhydride and/or unsaturated ester compound into a polyolefin to form a somewhat polar additive. The additive is then blended into a TPO, thereby increasing its polarity and improving its adhesion to coatings and adhesives. This approach is described, for example, in U.S. Pat. No. 3,873,643 to Wu et al., U.S. Pat. No. 3,856,889 to McConnell, U.S. Pat. No. 3,882,194 to Krebaum et al., U.S. Pat. No. 4,298,712 to Machonis et al., and DeVito et al., J. Polym. Sci., Polym. Chem. Edit. 2; 1335p (1984). Unfortunately, it has been found that TPOs modified with additives of this type often do not have sufficient adhesion to coatings and adhesives.
A variation of the foregoing approach is described by R. Ryntz et al., "Toward Achieving Directly Paintable TPO: Initial Paintability Studies", TPOs in Automotive, Executive Conference Management, Ann Arbor, Mich., (October 1995) and in U.S. Pat. Nos. 5,179,164 and 5,783,630 to Evans et al. In this approach, a modifier is prepared by grafting unsaturated anhydride, ester or acid monomers onto a polypropylene backbone, and reacting that grafted material with a polyetheramine. The modifier is then blended with a TPO to improve its paintability. Unfortunately, the approach suffers from the drawbacks of using relatively expensive aminated polyethers as raw materials. In addition, the modified polypropylene is often prepared in situ by feeding polypropylene, grafted polypropylene and polyetheramine into an extruder. This may lead to some problems with inconsistency in the product and volatilization of the amine in the extruder.
It would be desirable to provide an efficient means for improving the adhesion of polyolefins, and TPOs in particular, with a minimal adverse impact on its desirable physical properties.